Two cases are reported in which a rare hyperthyroidism appeared: in a female after radioiodine therapy for toxic multinodular goiter and in a male after spontaneous regression of a toxic adenoma. Both subjects showed a relapse of hyperthyroidism after a period of well-being lasting almost eight months in the first and three years in the second. Thyroid scans were consistent with an immunogenic hyper-thyroidism because there was a diffuse trapping of 131I in the thyroids while the previous autonomously functioning nodules became "cold". Serum TSH was undetectable, free thyroid hormones were increased, TgAb and TRAb were always normal in both patients, TPO became moderately positive only in the female. TRAb were evaluated only by radioimmunoassay. In these patients a diagnosis of Graves'-like disease was made because of the clinical and scintigraphic pattern. Moreover US did not reveal nodular areas different from those highlighted by scans. None of the subjects developed ophthalmopathy and/or dermopathy. Our remarks show that in particular subjects, genetically susceptible to autoimmunity, the release of antigenic materials secondary to destruction of thyroid nodules can trigger an autoimmune thyroid response resembling Graves' disease. Therefore all patients carrying autonomous nodules should be carefully evaluated for a possible autoimmune disposition before treatment and after admission. Radionuclide imaging is a simple, reliable, non invasive technique which can be applied in the evaluation of the etiology of the relapses.